A magnetron includes a cathode and anode arranged coaxially about a longitudinal axis, the anode defining a plurality of resonant cavities. The frequency of radiation generated by the magnetron is principally determined by the dimensions of the resonant anode cavities, especially their length in the axial direction and also, but not to such a significant extent, their radial lengths.
One method which is currently employed to provide frequency tuning involves use of an annular plate arranged over the anode cavities. The plate is vibrated so as to change its distance from the anode and hence alter the resonant frequency characteristics. In another method, prongs are inserted by a variable amount into the cavities to produce perturbations causing the output frequency to change.